1. Field of the Invention
This invention relates to ophthalmic lenses and, in particular, to multifocal ophthalmic lenses. More particularly, this invention relates to an improvement in phase plate optics as applied to ophthalmic lenses.
2. Description of the Related Art
This invention concerns ophthalmic lenses such as contact lenses, intraocular lenses (IOL's) and, more particularly, such lenses utilizing phase plate optics, such as phase plate bifocals and "tuned" Fresnel lenses making use of concentric annular zones. Such lenses generally follow the designs described, for example, by Allen L. Cohen in U.S. Pat. Nos. 4,210,391; 4,338,005; and 4,340,283 (hereinafter "the Cohen patents").
The lens design described in the Cohen patents provides that the radii "r.sub.n " of the annular and concentric zones are substantially proportional to .sqroot.n and that the zones are configured so as to direct light to more than one focal point. Such a lens design with phase plate optics allows lens constructions which are exceptionally thin. Contact lenses may be designed with phase plate optics in order to achieve a bifocal or multifocal effect. The specific chromatic properties of a phase plate may be incorporated in the design of a contact lens including a contact lens having multifocal properties.
There is also the need in multifocal lenses to have the capability of varying the intensity of light through a lens to accommodate pupil dilation and constriction. It is known that a pupil can vary from 3 mm to 6 mm, depending upon the level of ambient illumination. It would be desirable to be able to vary the distribution of energy between distance and near focal point according to the user's needs. For example, in dim illumination, the user of a contact lens will be typically engaged in distance viewing, such as driving an automobile. It would be desirable to have a contact lens which accommodates that condition.
Conversely, a user may seek to have maximum preference of light intensity to the near focus of the lens, yet would require a reasonable intensity of light for distance viewing. It would be desirable to have lenses that can be biased to a user's requirements for light intensity.
The nature of the light intensity problem is illustrated by reference to FIGS. 1 and 2. The lens portion depicted in FIG. 1 is a cross-sectional side view of a portion of a half-wave bifocal phase plate with the echelette depths h given by the equation: EQU h=.lambda./2(n'-n),
where:
.lambda.=wavelength of light; PA1 n'=refractive index of the contact lens; and PA1 n=refractive index of tear layer or eye. PA1 h=.lambda./2(n'-n); PA1 O&lt;k&lt;2, where k is an arbitrary number.
In FIG. 1 the individual amplitudes of light a.sub.0 and a.sub.1 are formed by the individual echelettes E. The total resultant amplitudes of light A.sub.0 and A.sub.1 formed at the 0.sup.th and 1.sup.st diffractive foci are also shown in FIG. 1. In this illustration, the equal length of vectors A.sub.0 and A.sub.1 demonstrates that the intensity of light is split equally between the two focal points. The intensities at the 0.sup.th order and the 1.sup.st order diffractive foci, I.sub.0 and I.sub.1, respectively, are given by: EQU I.sub.O =sinc.sup.2 (1/2) EQU I.sub.1 =sinc.sup.2 (1/2),
It is not necessary for a bifocal phase plate to split the incident light equally between its two diffractive foci when the vectors A.sub.0 and A.sub.1 are in parallel. This is shown in FIG. 2 of the cross-section of a portion of a bifocal phase plate with the echelette depths d given by the formula: EQU d=k.times.h,
where:
and
In the case of the FIG. 2 illustration, the intensity of light is not split equally between the two focal points. The intensities at the 0.sup.th order and 1.sup.st order diffractive foci, I.sub.0 and I.sub.1, respectively, of this example are derived from the following equations: EQU I.sub.0 =sinc.sup.2 (k/2) EQU I.sub.1 =sinc.sup.2 (1-k/2)
In this case, the amplitudes of light A.sub.0 and A.sub.1 are shifted in phase to parallel non-vertical aligned amplitudes produced by half-wave bifocal phase plate. The phase shift e is derived from the equation: EQU e=(1-k).pi./2
Though the current developments are significant improvements in the art, there is always a need to improve on the adaptability of the lenses to pupil-diameter variations and decentration. It is desirable to provide bifocal performance of a lens of the Cohen design with the feature that it can shift focused light from the distant to near focal points in coordination with the human eye's pupil, which normally constricts during near viewing.
It has been determined that contact lenses with phase plate optics may generate a few problems for the wearer. One is the glare that results from the non-optical edges of the steps between the annularly arranged echelettes that make up a phase plate and appear through wave interference as a disconcerting, intense light to the contact lens user.
Another potential problem stems from the need in soft contact lenses to have sufficient mobility when fit to the cornea to allow tear fluid exchange for cleansing the surface of the eye of metabolic waste. A further potential problem is the inability of the soft lens to move sufficiently during wearing to satisfy the just-described needed mobility.
The provision of a multiplicity of Fresnel echelettes in the annular zone plate arrangement of the Cohen lens design in a soft contact lens tends to limit the mobility of the lens. It would be desirable to incorporate into the design of such lenses sufficient mobility that the lens has the capacity of moving about 0.5 to about 1 millimeter of distance during wearing. This would enhance the ability of the lens to allow management of the buildup of metabolic waste under the lens.
It is another feature of this invention to provide a multifocal contact lens design encompassed within the annular arrangement of the Cohen patents which minimizes the effects of glare from the non-optical edges and/or possesses the requisite mobility during use, as characterized above.